In replacing a hip joint, the head of the femur is removed along with the ball. The femur is shaped and prepared for receiving the prosthesis so that the artificial joint will closely approximate the natural hip.
The modular artificial joint has three elements which replace the natural hip. The intramedullary rod is inserted into the end of the femur. The entire prosthesis is supported by this connection between the rod and the femur. The upper portion of the rod which extends out of the femur is fitted into a trochanter element which is shaped like the removed broad head of the femur which it replaces. This element, along with the rod, is used to adjust the length of the prosthesis to approximate the natural length of the femur. A neck element is inserted into the trochanter element and carries an angular extension onto which the ball joint will be fixed. All these elements have a central bore and are permanently secured together by a bolt which is inserted into the neck element, extends through the trochanter element, and is threaded into the upper end of the rod. In some cases, the intramedullary rod may be attached to the bone with bone cement while, in other cases the cement is omitted.
When the cement is omitted, the placement and fixation of the intramedullary rod becomes more critical to pain free usage of the prosthesis. Further, it is most important that the intramedullary rod not be disturbed after insertion since this would corrupt the union between the rod and the interior of the femur.
In order to maintain the original union between the femur and the intramedullary rod, modular prosthesis have been developed to allow rotational adjustment of the several parts or elements about the rod during the placement of the prosthesis to more closely reproduce the natural structure of the hip. The modular concept also allows the selection of different sized elements, before or during surgery, to more closely approximate the natural joint.
With the advantage of flexibility gained by modular prosthesis, there comes the requirement that there be no movement between the several parts or elements after implantation. These movements may cause misalignment of the joint resulting in increased pain, trauma to the joint and, even, dislocation of the joint.